No, he was letting his imagination run away with him.

The other possibility was that his imagination had gone and run away with him.

If a person isn't careful, their imagination can really run away with them.

This time my imagination may really be running away with me.

"My imagination was running all over the place," he said.

He must have let his imagination run away with him.

On the other hand, he might be letting his imagination run too free.

She had no idea what to expect but her imagination ran wild.

His fertile imagination had been running away with him again.

I think you've let your imagination run away with you a bit there.